Stack Value File : Custom Microarchitecture for the Stack

1. Stack Value File : Custom Microarchitecture for the Stack Hsien-Hsin Lee Mikhail Smelyanskiy Chris NewburnGary Tyson University of Michigan Intel Corporation

2. Agenda • Organization of Memory Regions • Stack Reference Characteristics • Stack Value File • Performance Analysis • Conclusions

3. Memory Space Partitioning max mem • Based on programming language • Non-overlapped subdivisions • Split code and data ÞI-cache & D-cache • Split data into regions • Stack (¯) • Heap (­) • Global (static) • Read-only (static) reserved Stack grows downward Protected Heap grows upward Global Static Data Region Code Region Read-only data reserved min mem

4. Memory Access Distribution • SPEC2000int benchmark (Alpha binary) • 42% instructions access memory

5. Access Method Breakdown 86% of the stack references use ($sp+disp)

6. Morphing $sp-relative References • Morph $sp-relative references into register accesses • Use a Stack Value File (SVF) • Resolve address early in decode stage for stack-pointer indexed accesses • Resolve stack memory dependency early • Aliased references are re-routed to SVF

7. Stack Reference Characteristics • Contiguity • Good temporal and spatial locality • Can be stored in a simple, fast structure • Smaller die area relative to a regular cache • Less power dissipation • No address tag need for each datum

8. Stack Reference Characteristics • First touch is almost always a Store • Avoid waste bandwidth to bring in dead data • A register write to the SVF • Deallocated stack frame • Dead data • No need to write them back to memory

9. Baseline Microarchitecture Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) ArchRF ReOrder Buffer

10. Microarchitecture Extension Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset ArchRF Max ReOrder Buffer Hash SP Stack SP Value File interlock

11. Microarchitecture Extension stq $r10, 24($sp) TOS Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset ArchRF Max ReOrder Buffer Hash SP Stack SP Value File interlock

12. Microarchitecture Extension stq $r10, 24($sp) TOS Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset 3 ArchRF Max ReOrder Buffer Hash SP Stack SP Value File interlock

13. Microarchitecture Extension stq $r10, 24($sp) $r35  ROB-18 TOS Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset ArchRF Max ReOrder Buffer Hash SP Stack SP Value File interlock

14. Microarchitecture Extension stq $r10, 24($sp) $r35  ROB-18 TOS Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset ArchRF Max ReOrder Buffer Hash SP Stack SP Value File interlock

15. File Value Microarchitecture Extension stq $r10, 24($sp) $r35  SVF3 TOS Issue Execute Commit Ld /St Dispatch Fetch Decode MOB Unit DecoderQ Reservation Station / LSQ Reg Decoder Instr -Cache Renamer Func Unit ( RAT) Morphing Pre-Decode offset ArchRF Max ReOrder Buffer Hash SP Stack SP interlock

16. Why could SVF be faster ? • It reduces the latency of stack references • It effectively increases the number of memory port by rerouting more than ½ of all memory references to the SVF • It reduces contention in the MOB • More flexibility in renaming stack references • It reduces memory traffic

17. Simulation Framework Simple Scalar (Alpha binary), OOO model

18. Speedup Potential of SVF • Assume all references can be morphed • ~30% speedup for a 16-wide with dual-ported L1

19. SVF Reference Type Breakdown • 86% stack references can be morphed • Re-routed references enter normal memory pipeline

20. Comparison with stack cache • (R+S) : Regular and Stack or SVF cache ports

21. Memory Traffic • SVF dramatically reduces memory traffic by many order of magnitude. • For gcc, ~28M (Stk$  L2) reduced to ~86K (SVF  L1). • Incoming traffic is eliminated because SVF does not allocate a cache line on a miss. • Outgoing traffic consists of only those words that are dirty when evicted (instead of entire cache lines).

22. SVF over Baseline Performance • (R+S) : Regular and SVF cache ports

23. Conclusions • Stack references have several unique characteristics • Contiguity, $sp+disp, first reference store, frame deallocation. • Stack Value File • a microarchitecture extension to exploit these characteristics • improves performance by 24 - 65%

24. Questions & Answers

25. That's all, folks !!! http://www.eecs.umich.edu/~linear

26. Backup Foils

27. Stack Depth Variation

28. Cumulative % Offset in Bytes (Log scale) Offset Locality of Stack • Cumulative offset within a function call • Avg: 3b - 380b • >80% offset within“400b” • >99% offset within“8Kb”

29. Conclusions • Stack reference features • Contiguity • No dirty writeback when stack deallocated • Stack Value File • Fast indexing. • Alleviate multi-porting L1 cache. • Smaller, No tags, and less power. • Exploiting ILP